Polyvinylidene fluoride resins (PVDF) have been used as materials for porous resin membranes which are excellent in chemical resistance, heat resistance and mechanical properties.
In connection with these porous polyvinylidene fluoride resin membranes, for example, U.S. Pat. No. 5,022,990 discloses a process for producing a porous polyvinylidene fluoride membrane which comprises blending a polyvinylidene fluoride resin with an organic liquid and an inorganic particulate material and subsequently melt molding the resulting blend in the form of a hollow fiber, tubular or flat membrane. The porous membrane prepared with this process has a uniform, three-dimensional, network structure. Particularly, an example of the U.S. patent describes a porous hollow fiber membrane having an inside diameter of 1.10 mm and a wall thickness of 0.45 mm. However, if the inside diameter is increased to larger than this value during the process, the membrane pressure resistance lowers because the membrane wall stretches thin as the inside diameter increases. On the other hand, if both the inside diameter and the wall thickness are increased to larger than these values during the process, then there is the problem that the resulting porous hollow fiber membrane has such a decrease in water flux as to be impractical.
As described above, it has heretofore been impossible for any conventional technique to provide a porous hollow fiber polyvinylidene fluoride resin membrane which has excellent pressure resistance and high water flux despite having a large inside diameter and which is therefore suitable especially for treatment of highly viscous liquids.